1. Field of the Invention
This invention relates generally to rotary drill bits used in drilling subterranean wells and, more specifically, to rotary drill bits employing a torque limiting device allowing the drill string to rotate relative to the crown of the bit when a predetermined reactive torque is experienced by the crown of the drill bit.
2. State of the Art
The equipment used in drilling operations is well known in the art and generally comprises a drill bit attached to a drill string, including drill pipe and drill collars. A rotary table or other device such as a top drive may be employed to rotate the drill string, resulting in a corresponding rotation of the drill bit. The drill collars, which are heavier and stiffer than drill pipe, are normally used on the bottom part of the drill string to add weight to the drill bit. The weight of these drill collars assists in stabilizing the drill bit against the formation at the bottom of the borehole, causing it to drill when rotated. Too much weight on bit (WOB), however, may cause the drill bit to stall.
Downhole motors may also be employed to rotate the drill bit and include two basic components: a rotor, which is a steel shaft shaped in the form of a spiral or helix, and a stator, which is a molded rubber sleeve in a rigid tubular housing, that forms a spiral passageway to accommodate the rotor. When the rotor is fitted inside the stator, the difference in geometry between the two components creates a series of cavities through which drilling fluid is pumped. In doing so, the fluid displaces the rotor, forcing it to rotate as the fluid continues to flow between the rotor and the stator. An output shaft connected to the rotor transmits its rotation to the bit.
A typical rotary drill bit includes a bit body secured to a steel shank having a threaded pin connection for attaching the bit body to the drill string or the output shaft of a downhole motor and a crown comprising that part of the bit fitted with cutting structures for cutting into an earth formation. Generally, if the bit is a fixed-cutter or so-called xe2x80x9cdragxe2x80x9d bit, the cutting structure includes a series of cutting elements made of a super-abrasive substance, such as polycrystalline diamond, oriented on the bit face at an angle to the surface being cut. On the other hand, if the bit has rotating cutters such as on a tri-cone bit, each cone independently rotates relative to the body of the bit and includes a series of protruding teeth, which may be integral with the cone or comprise separately formed inserts.
The bit body of a drag bit is generally formed of steel or a matrix of hard particulate material such as tungsten carbide infiltrated with a binder, generally of copper-based alloy. In the case of steel body bits, the bit body is usually machined from round stock to the shape desired, usually with internal watercourses for delivery of drilling fluid to the bit face. Topographical features are then defined at precise locations on the bit face by machining, typically using a computer-controlled, five-axis machine tool. For a steel body bit, hardfacing may be applied to the bit face and to other critical areas of the bit exterior, and cutting elements are secured to the bit face, generally by inserting the proximal ends of studs on which the cutting elements are mounted into apertures bored in the bit face. The end of the bit body opposite the face is then threaded, made up and welded to the bit shank.
In the case of a matrix-type drag bit body, it is conventional to employ a preformed so-called bit xe2x80x9cblankxe2x80x9d of steel or other suitable material for internal reinforcement of the bit body matrix. The blank may be merely cylindrical and tubular, or may be fairly complex in configuration and include protrusions corresponding to blades, wings or other features on the bit face. Other preform elements comprised of sand, or in some instances tungsten carbide particles, in a flexible polymeric binder may also be employed to define internal watercourses and passages for delivery of drilling fluid to the bit face, as well as cutting element sockets, ridges, lands, nozzle displacements, junk slots and other external topographic features of the bit. The blank and other preforms are placed at appropriate locations in the mold used to cast the bit body before the mold is filled with tungsten carbide. The blank is bonded to and within the matrix upon cooling of the bit body after infiltration of the tungsten carbide with the binder in a furnace, and the other preforms are removed once the matrix has cooled. The threaded shank is then welded to the bit blank. The cutting elements (typically diamond, and most often a synthetic polycrystalline diamond compact, or PDC) may be bonded to the bit face by the solidified binder subsequent to furnacing of the bit body. Thermally stable PDCs, commonly termed xe2x80x9cTSPsxe2x80x9d, may be bonded to the bit face by the furnacing process or may be subsequently bonded thereto, as by brazing, adhesive bonding, or mechanical affixation.
In order for the cutting elements to properly cut the formation during a drilling operation, considerable torque is required to generate the necessary rotational force between the cutting elements and the formation under a WOB substantial enough to ensure an adequate depth of cut. The resultant or reactive torque on the bit from formation contact is translated through the drill string and must be overcome by the means used to rotate the drill string, such as a rotary table, top drive, or downhole motor. In some instances, such as drilling through harder formations, the resultant torque may result in the winding up and sudden release of the drill string under torque, manifested as so-called xe2x80x9cslapsxe2x80x9d of the drill string at the rotary table. In other instances, torque may be sufficient to actually stop the bit from rotating. The rotary table may continue to rotate the drill string for some time, in effect xe2x80x9ctwistingxe2x80x9d the drill string and placing the bit under very high torque loads before an operator realizes that the bit is no longer rotating. This problem is of particular concern with drag bits, due to direct engagement of the formation by the fixed PDC cutters, but also manifests itself with rock bits. If such a condition occurs and the rotary table continues to rotate, the drill string, the bit and/or components thereof may be damaged, or the drill string may even part under the torque load. If failure of the drill string occurs, the portion of the drill string above the break must be removed from the wellbore. A xe2x80x9cfishingxe2x80x9d assembly inserted into the wellbore is then normally employed in an attempt to retrieve the remainder of the drill string. If retrieval is impractical or unsuccessful, a new drilling assembly must be deflected, xe2x80x9csidetracked,xe2x80x9d or steered around the xe2x80x9cfish.xe2x80x9d Any such scenario adds to the cost of production and results in down-time of the drilling operation while the remainder of the broken drill string is xe2x80x9ctrippedxe2x80x9d from the wellbore and replaced with other bottom hole assemblies.
When a downhole motor is being used to rotate the drill bit, a sudden rise in surface pressure of the drilling fluid may indicate that the motor has stalled. While other conditions may cause a rise in fluid pressure, such as a clogged motor or plugged nozzles, if the motor stalls because the bit is no longer rotating due to excessive torque on the bit and is maintained in a stalled condition, the elastomeric stator lining may be damaged, preventing a proper interface between the stator and the rotor, thus requiring the motor to be tripped out of the wellbore and replaced. At the least, the bottomhole assembly, including the motor, must be pulled off-bottom and drilling and circulation recommended to start the motor before the formation is re-engaged by the bit.
In addition to damage to drill strings and bits, directional drilling presents its own set of problems when excessive torque is applied to the drill bit. A directional well must intersect a target that may be several miles below the surface location of the drilling rig, and laterally offset therefrom. In order to reach the target, the wellbore must be directed or steered along a predetermined trajectory. The trajectory of the bit is typically determined by the tool face orientation (TFO), which must be maintained during drilling in order to maintain the trajectory of the wellbore toward the desired target. If the TFO shifts due to a stalled drill bit, the drilling must stop and a new TFO set as a reference point for the direction of drilling. While a shift in TFO is quickly manifested to the operator due to the essentially real-time nature of the MWD (measurement while drilling) mud-pulse transmissions, nonetheless, loss of TFO and resetting thereof results in considerable reduction in the overall rate of penetration (ROP) of the drilling assembly.
It would thus be advantageous to provide a drill bit assembly that includes a torque limiting device that is either an integral part of the bit construction or is attached near the bit between the drill bit and the drill string, or is positioned between the downhole motor and the drill bit.
According to the present invention, a torque limiting device is provided that allows the drill string to rotate relative to the cutting structure of the bit at a predetermined torque placed on the cutting structure of the bit. The torque limiting device may be incorporated into the structure of the bit itself, be a separate structure attached to a drill bit, or be near-bit positioned between the drill string and the bit. In any case, the torque limiting device prevents movement of the cutting structure relative to the drill string during normal operation. When a predetermined torque is applied to the cutting structure of the bit, the torque limiter allows the drill string to rotate relative to the stationary cutting structure until the torque is decreased below the predetermined level, typically by backing off the drill string to decrease the WOB.
In a preferred embodiment having the torque limiting device as an integral part of a drill bit, the fixed-cutter bit is comprised of a crown for providing a cutting face to which a plurality of cutting elements may be attached and a shank for supporting the crown and attaching the crown to a drill string. The crown has a substantially cylindrical internal chamber sized and shaped to mate with and effectively cap the proximal end of the shank, which also has a generally cylindrical configuration. The shank and the crown fit together in a snug arrangement without inhibiting rotational movement between the crown and the shank.
In one preferred embodiment, around the perimeter of the shank are a number of recesses positioned to match corresponding recesses formed in the wall of the internal chamber of the crown. A biasing member comprised of a resilient material or a spring is placed in each recess formed in the shank. A retaining member, preferably made of a hard material such as steel, is subsequently placed on top of (radially outboard of) each of the biasing members. When the shank and crown are assembled together longitudinally, the retaining member compresses the biasing member and is forced by the wall of the internal chamber of the crown into the recess formed in the shank. The lower portion of the retaining member may be tapered to facilitate assembly of the torque limiting device. When the shank and crown are completely engaged, the biasing member forces the retaining member into the recess in the internal chamber wall.
If sufficient torque is applied to the crown of the bit, the retaining member is forced against the biasing member out of the recess in the internal chamber wall of the crown. The shank can then rotate relative to the crown. If a single retaining member and recess are utilized as part of the torque limiting device, the shank will make a complete revolution before the retaining member can reengage the recess. If the torque is still sufficient, the shank will continue to rotate until the torque is sufficiently decreased and the retaining member is realigned with the recess. Preferably, there is more than one retaining member and more than one recess spaced around the perimeter of the shank. Thus, the retaining member or members may reengage with other recesses, depending on when the torque is sufficiently lowered. In addition, the retaining member may be longitudinally oriented or oriented at some angle relative to the bit axis. Engagement or disengagement of the retaining member or members with the recesses manifests itself as vibrations on the rig floor, alerting the driller to reduce WOB.
In another preferred embodiment where the torque limiting device is part of the drill bit itself, the crown is securely attached to a substantially cylindrical bit blank. The blank and the shank are then attached in a manner similar to the aforementioned embodiment, including the torque limiting feature. Such a configuration may be necessary if the crown is comprised of a relatively brittle material, such as tungsten carbide, where forming recesses therein and engaging and reengaging a retaining member may cause the crown to crack. Thus, the blank is preferably formed of a more ductile material and the crown of a more abrasion-resistant material, with the recesses necessary for engagement of the retaining member formed in the blank.
In either of the aforementioned embodiments, a standardized shank could be manufactured to accommodate a variety of crown and/or cutter sizes and configurations. In yet another embodiment, the crown is configured to be inserted into the proximal end of the shank with the proximal end of the shank having a substantially cylindrical chamber formed therein to mate with the distal end of the crown. The torque limiting device of the aforementioned embodiments is utilized in a substantially similar manner to limit the torque that may be applied to the bit crown.
In still another preferred embodiment where the torque limiting device is part of the bit itself, a pair of bands is positioned between the shank and the blank with one band attached to each. The bands maintain relative position due to a frictional interference fit but can slide relative to one another if a predetermined torque is applied to the crown of the bit. In addition, the bands may have various orientations including vertical, horizontal, or any angle therebetween. Moreover, one or both of the bands may be comprised of a resilient material, such as synthetic elastomers, and the band material may be filled with particles or fibers of asbestos or other brake-material compounds. The location of the bands may be sealed from wellbore fluids, or the band materials may be selected to operate in the wellbore environment. Such a torque limiting device would act in a clutch-like manner where the bands remain in stationary relationship, so long as the force between them caused by torque on the crown does not exceed the static coefficient of friction between the bands. Moreover, the torque limiting device would have equal utility for tri-cone bits, as well as coring or other bits used in rotational-type drilling.
In yet another preferred embodiment, the torque limiting device includes a plurality of load-driven rollers (clutch rollers) that allows rotational movement when a predetermined torque or load is placed on the cutting structure of the bit.
In another preferred embodiment, a ratchet-type torque limiter may be comprised of two substantially concentric rings of similar or dissimilar materials, each having teeth or projections in engaging contact with one another that disengage when a predetermined torque is applied to the cutting structure of the bit.
In an alternate embodiment where the torque limiting device of the present invention is separate from the bit, the device couples a typical drill bit to a drill string and/or downhole motor. The torque limiting device includes connecting structures, such as threads, at both ends, one for attaching the device to the bit and one for attaching it to the drill string. The device may be formed as part of a downhole motor, or as a near-bit sub. Similar to the construction of the drill bit embodiments, the torque limiter may be comprised of two connecting structures that are fitted together in a male-female interconnection and held together by retaining members engaged in recesses formed in the internal wall of one connector. If sufficient torque is applied to the bit by the formation, the torque limiting device will allow the drill string to rotate relative to the bit.
As will be recognized, when the retaining members are disengaged from their respective recesses, the two connecting structures need not be axially mechanically attached to one another except for frictional forces applied by the retaining members on the internal wall of one connecting structure. Because the bit is being forced into the bottom of the wellbore, however, the two connecting structures are held together by the weight of the drill string. Thus, the two connecting structures will not become separated. The same is true for the embodiments where the torque limiting device is part of the bit construction. However, as required, additional structures as known in the art may be employed to help the two connecting structures remain secured together against longitudinal tensile forces encountered when tripping out of the wellbore.
It will be recognized by those skilled in the art that in any of the aforementioned embodiments, the configurations of the retaining and biasing members may vary. For example, the retaining member may simply be spherically shaped, cylindrically shaped, wedge shaped or otherwise suitably shaped including combinations thereof. Moreover, the retaining members may be biased by a segment of resilient material, a coil-type spring, a leaf spring, a belleville spring, or other means known in the art.
As noted above, a torque limiting device, in accordance with the present invention, will reduce the possibility of bit damage from excessive torque and will quickly signal the drilling operator through vibrations or shock waves that excessive torque is being applied to the drill bit.